Exposure apparatus

ABSTRACT

In an exposure apparatus employing an over filled optical system, the light quantity distribution on a scanning plane is kept nearly constant for a plurality of scanning light quantities. It selects the light quantity of the light beam irradiated onto the photosensitive body from a plurality of levels, and sets the light quantity selected. According to the light quantity, it selects one of a plurality of correction current profiles, and supplies a light source with a current passing through the correction based on the correction current profile selected. Since the light quantity of the light beam irradiated onto the photosensitive body is corrected by the correction current, the light quantity of the light beam on the photosensitive body becomes nearly constant in the scanning direction.

This application is a divisional of application Ser. No. 11/673,666, nowallowed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exposure apparatus, and moreparticularly to an exposure apparatus for exposing a photosensitive bodyby carrying out the main scanning of the photosensitive body bydeflecting a light beam emitted from a light source.

2. Description of Related Art

Exposure apparatuses have conventionally been employed in image formingapparatuses such as copying machines, fax machines and laser printers.Such an exposure apparatus carries out the main scanning of the surfaceof a drum-like photosensitive body with a laser beam corresponding toimage information using a light deflecting means such as a polygonmirror, and carries out subscanning by rotating the photosensitive bodyin the subscanning direction. Thus, an electrostatic latent imagecorresponding to the image information is formed on the photosensitivebody.

Recently, since the printing speed of the image forming apparatuses hasincreased, the polygon mirror rotation rate in an under filled opticalsystem is reaching its limit. Accordingly, over filled optical systemshave been employed. In the over filled optical system, the laser beam isincident upon a plurality of reflection planes of a polygon mirrorsimultaneously by making the scanning direction length of the individualreflection planes of the polygon mirror smaller than the scanningdirection diameter of the laser beam incident upon the polygon mirror.Compared with the under filled optical system, the over filled opticalsystem can greatly reduce the scanning direction length of thereflection planes required for producing a prescribed magnitudespotlight on the photosensitive body. In addition, the over filledoptical system can provide a greater number of reflection planes to thepolygon mirror of the same diameter than the under filled opticalsystem. Thus, the over filled optical system can rotate the polygonmirror at a lower speed than the under filled optical system. Besides,the over filled optical system can employ a polygon mirror driving unitwith smaller power consumption.

In the over filled optical system, however, each reflection plane of thepolygon mirror reflects part of the incident laser beam as describedbefore. Accordingly, the light quantity incident upon the reflectionplane of the polygon mirror varies depending on the angle of the polygonmirror. FIG. 10 is a graph showing the conditions. The distribution ofthe light quantity incident upon the photosensitive body is asillustrated in FIG. 10. More specifically, the light quantity takes thehighest value at the center of the photosensitive body, and graduallydecreases toward the two edges. The variations in the light quantitydistribution can have adverse effect on the image quality.

To solve the problem, Japanese patent application laid-open No.09-197316/1997 discloses a technique that measures the light quantitiesat points on a scanning plane on the photosensitive body, measuresfluctuations on the scanning plane, and stores the fluctuations.According to the stored values, it corrects the light quantity of thelaser beam, thereby controlling the light quantity on the scanning planein such a manner as to become nearly constant.

However, to enrich media, to improve efficiency of developing resources,or to speed up product development, it is required to apply a singleexposure apparatus to a variety of apparatuses such as a laser printeror MFP (multifunction peripheral). Thus, it becomes necessary forindividual exposure apparatuses to cope with various specifications.

In particular, to cope with the various specifications, this type of theexposure apparatus has a problem of the rotation rate of the polygonmirror and the laser light quantity. As for the rotation of the polygonmirror, such an apparatus that detects the rotation rate according tothe detection period of a horizontal synchronizing signal detectingmeans and controls the rotation will be able to vary the rotation rateinexpensively and easily.

As for the laser light quantity also, the conventional under filledscanner can easily vary the light quantity by varying the target lightquantity of APC (automatic power control). In contrast, the over filledoptical system must carry out the light quantity correction during asingle scanning.

However, as described in Japanese Patent Application Laid-open No.09-197316/1997, the conventional apparatuses do not cope with aplurality of scanning light quantities. Thus, an exposure apparatuscapable of coping with various specifications has not been implemented.

SUMMARY OF THE INVENTION

The present invention is implemented to solve the foregoing problems. Itis therefore an object of the present invention to provide an exposureapparatus capable of coping with a plurality of scanning lightquantities in the over filled optical system.

Another object of the present invention is to provide an exposureapparatus capable of carrying out light quantity correction in such amanner as to make the light quantity distribution nearly constant in asingle scanning direction in response to a plurality of scanning lightquantities.

Still another object of the present invention is to provide an exposureapparatus capable of improving the accuracy of the light quantitycorrection in accordance with target scanning light quantity.

Still another object of the present invention is to provide an exposureapparatus capable of easily coping with an increasing number of targetscanning light quantities.

According to one aspect of the present invention, there is provided anexposure apparatus comprising: a light source for emitting a light beamwith a light quantity corresponding to a current amount supplied; arotating polygon mirror with a plurality of reflection planes, saidrotating polygon mirror rotating in a manner that the light beam emittedfrom said light source and reflected off the reflection plane scans on aphotosensitive body; beam magnifying means for expanding the light beamemitted from said light source in a manner that the light beam isirradiated onto said rotating polygon mirror with exceeding beyond awidth of each reflection plane of said rotating polygon mirror in ascanning direction; correction current profile memory means for storinga plurality of correction current profiles that will provide thescanning direction with a nearly fixed light quantity of the light beamirradiated onto said photosensitive body; setting means for selectingfrom a plurality of levels a target light quantity of the light beamirradiated onto the photosensitive body, and for setting the targetlight quantity selected; and current supplying means for selecting oneof the plurality of correction current profiles stored in saidcorrection current profile memory means in response to the target lightquantity set by said setting means, and for supplying said light sourcewith a current passing through correction based on the correctioncurrent profile selected.

According to another aspect of the present invention, there is providedAn exposure apparatus comprising: a light source for emitting a lightbeam with a light quantity corresponding to a current amount supplied; arotating polygon mirror with a plurality of reflection planes, saidrotating polygon mirror rotating in a manner that the light beam emittedfrom said light source and reflected off the reflection plane scans on aphotosensitive body; beam magnifying means for expanding the light beamemitted from said light source in a manner that the light beam isirradiated onto said rotating polygon mirror with exceeding beyond awidth of each reflection plane of said rotating polygon mirror in ascanning direction; correction current profile memory means for storinga correction current profile that will provide the scanning directionwith a nearly fixed light quantity of the light beam irradiated ontosaid photosensitive body; setting means for selecting from a pluralityof levels a target light quantity of the light beam irradiated onto thephotosensitive body, and for setting the target light quantity selected;and current supplying means for generating correction informationcorresponding to the correction current profile stored in saidcorrection current profile memory means in response to the target lightquantity set by said setting means, and for supplying said light sourcewith a current passing through correction in accordance with thecorrection information generated, at timing in response to thecorrection information.

According to the present invention, the light quantity distribution inthe scanning direction can be made constant in response to a pluralityof photosensitive body irradiation light quantities.

In addition, in accordance with a plurality of photosensitive bodyirradiation light quantities and a plurality of scanning speeds, thelight quantity correction can be performed which can provide lightquantity distribution nearly constant in the scanning direction.

Furthermore, in response to set light quantities on the photosensitivebody, and in accordance with a plurality of photosensitive bodyirradiation light quantities and a plurality of scanning speeds, thelight quantity correction can be performed which can provide lightquantity distribution nearly constant in the scanning direction at highaccuracy.

Moreover, since the number of data in a nonvolatile memory can be madeless than a predetermined number of light quantities required, a memorywith a smaller capacity can be used.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an exposure apparatus inaccordance with the present invention;

FIG. 2 is a block diagram showing a configuration of the exposureapparatus in accordance with the present invention;

FIG. 3 is a timing chart illustrating an embodiment 1 in accordance withthe present invention;

FIG. 4 is a diagram explaining light quantity correction of the exposureapparatus in accordance with the present invention;

FIG. 5 is a block diagram explaining a control block of the exposureapparatus in accordance with the present invention;

FIG. 6 is a diagram explaining light quantity correction of the exposureapparatus of the embodiment 1 in accordance with the present invention;

FIG. 7 is a flowchart illustrating the operation of the embodiment 1 inaccordance with the present invention;

FIG. 8 is a table explaining the light quantity correction of theexposure apparatus of the embodiment 1 in accordance with the presentinvention;

FIG. 9 is a table explaining the light quantity correction of theexposure apparatus of an embodiment 2 in accordance with the presentinvention;

FIG. 10 is a diagram explaining light quantity distribution of an overfilled optical system;

FIG. 11 is a table explaining Vref voltage setting of the embodiment 2in accordance with the present invention;

FIG. 12 is a diagram explaining light quantity correction of theexposure apparatus of the embodiment 2 in accordance with the presentinvention;

FIG. 13 is a flowchart illustrating the operation of the embodiment 2 inaccordance with the present invention;

FIG. 14 is a table explaining the light quantity correction data of theexposure apparatus of the embodiment 2 in accordance with the presentinvention;

FIG. 15 is a diagram explaining blocks of the exposure apparatus of anembodiment 3 in accordance with the present invention;

FIG. 16 is a table explaining an example of the light quantitycorrection of the exposure apparatus of the embodiment 3 in accordancewith the present invention;

FIG. 17 is a table explaining another example of the light quantitycorrection of the exposure apparatus of the embodiment 3 in accordancewith the present invention; and

FIG. 18 is a table explaining still another example of the lightquantity correction of the exposure apparatus of the embodiment 3 inaccordance with the present invention.

DESCRIPTION OF THE EMBODIMENTS

The invention will now be described with reference to the accompanyingdrawings.

Embodiment 1

FIG. 1 is a diagram explaining an exposure apparatus in accordance withthe present invention.

In FIG. 1, the reference numeral 1 designates a laser light source, 2designates a collimator lens, 3 designates a diaphragm, 4 designates amain scanning expander lens, 5 designates a cylindrical lens, and 7designates a reflection mirror. The reference numeral 8 designates apolygon mirror, 9 designates an fθ lens, and 10 designates an anamorphicaspheric lens having power primarily in the subscanning direction.

The laser light radiated from the laser light source 1 is made nearlyparallel through the collimator lens 2, and undergoes beam regulation bythe diaphragm 3. The beam is made divergent rays in the main scanningdirection through the main scanning expander lens 4. In addition, thelaser light is condensed only in the subscanning direction through thecylindrical lens 5, is reflected by the reflection mirror 7, passesthrough the fθ lens 9, and is condensed linearly near the reflectionplanes of the polygon mirror 8. The condensed laser light is reflectedby the polygon mirror 8. In this case, rotating at a constant rate, thepolygon mirror 8 deflects the laser light.

The deflected laser light is incident upon the fθ lens 9 with fθcharacteristics again to condense the ray bundle in the main scanningdirection. The ray bundle passing through the fθ lens 9 is condensed inthe subscanning direction by the anamorphic aspheric lens 10 havingpower in the subscanning direction, and forms a spot on thephotosensitive body drum not shown via a reflecting mirror 11. The spotscans the photosensitive body drum in the axial direction to carry outthe main scanning. The reference numeral 12 designates a reflectingmirror mounted on the edge in the main scanning direction. The referencenumeral 13 designates a horizontal synchronization detecting element (BDsensor) provided for determining the write start timing of an image. Thehorizontal synchronization detecting element 13 receives the laser lightfrom the laser light source 1 obtained via the reflecting mirror 12, andoutputs a signal responding to the horizontal synchronization timing.

FIG. 2 is a block diagram showing the control block of the exposureapparatus.

The semiconductor laser 1 has a semiconductor laser diode (LD) 21 and aphotodiode (PD) 22 placed near the LD 21. The PD 22 receives the laserbeam emitted from the LD 21, and outputs a current corresponding to thelight quantity of the laser beam received. The LD 21 has its anodeconnected to a power supply Vcc, and its cathode connected to the LDOterminal of a laser driver IC 27 which is a laser control means. The PD22 has its cathode connected to the power supply Vcc, and its anodeconnected to the PD terminal of the laser driver IC 27. A resistor R1 isconnected between the power supply Vcc and the RO terminal of the laserdriver IC 27. A resistor R3 is connected between a GND and the RMterminal of the laser driver IC 27. A capacitor C1 is connected betweenthe GND and the VCH terminal of the laser driver IC 27. A resistor R2 isconnected between the GND and the RS terminal of the laser driver IC 27.

An exposure apparatus control section 28 carries out the control of theAPC and laser switching. The exposure apparatus control section 28 has aCPU for carrying out the control, and a memory unit including thecontrol procedure of the CPU (inclusive of the control procedure asshown in FIG. 7 and FIG. 13) and a workspace of the CPU. Further theexposure apparatus control section 28 has a light quantity settingsection. The light quantity setting section can be set a plurality oflight quantities. The light quantity setting section selects one of aplurality of light quantities in accordance with a designation from aimage forming apparatus side and sets it.

The exposure apparatus control section 28 delivers the control signaland video signal to the laser driver IC 27. The output of a lightquantity correcting section 29 is supplied to a constant current circuit30 which converts it to a constant current. The constant current issupplied to the RS terminal of the laser driver IC 27 in such a manneras to maintain the light quantity on the scanning line at nearlyconstant on the photosensitive body drum not shown.

Next, a single scanning operation of the exposure apparatus controlsection 28 will be described with reference to FIG. 3.

In period (1), the exposure apparatus control section 28 instructs thelaser driver IC 27 to carry out APC so that the laser driver IC 27performs APC emission. Thus, the laser driver IC 27 has the LD 21 emitlight, thereby causing a current to flow through the PD 22 in responseto the emitted light quantity. The current flows from the RM terminal tothe resistor R3. During the APC emission, the current of the LD 21 isadjusted in such a manner that the voltage at the RM terminal becomes adesired value. After the adjustment, the laser driver IC 27 carries outthe current-to-voltage conversion of the current of the LD 21, andsupplies the voltage to the VCH terminal. If an output (Low) from the BDsensor is obtained during the APC emission, the laser driver IC 27terminates the APC emission immediately (or after a predetermined timeperiod), and makes a transition to a mask state.

In period (2), the exposure apparatus control section 28 instructs thelaser driver IC 27 to carry out mask control. According to theinstruction, the laser driver IC 27 halts the emission of the LD 21, andsupplies the resistor R1 and resistor R2 with a constant currentdetermined by the voltage at the VCH terminal. The purpose of thecurrent is to quicken the start-up of the LD 21. The duration of theperiod (2) is determined according to the time from the input of the BDsensor signal to the beginning of writing the image. It varies inaccordance with paper size and with/without a border.

In period (3), the exposure apparatus control section 28 instructs thelaser driver IC 27 to carry out video input. Thus, the laser driver IC27 carries out the laser emission in response to image data (videosignal). The laser driver IC 27 performs switching in such a manner thatwhen the video input is True, the current flows through the LD 21, andwhen it is False, the current flows through the resistor R1.

In period (4), the mask state comes again. As for the timing, it variesin accordance with paper size and with/without a border as in theprevious period (2).

With repeating the foregoing periods, the APC and BD detection arecarried out at each scanning. Thus, the electrostatic latent image isformed on the photosensitive body drum.

Next, the light quantity correction will be described.

FIG. 5 is a block diagram showing a configuration of the light quantitycorrecting section 29.

The light quantity correcting section 29 has a logic section 51, aninternal memory 52 including a nonvolatile memory and a variety ofsetting registers, and a D/A converter (converting section) 53. Thelogic section 51 receives from the exposure apparatus control section 28a trigger (TR) signal, operating clock (SCLK) signal, data out (DO)signal, data communication clock (SCK) signal, and select (CS) signal.On the other hand, the logic section 51 supplies the exposure apparatuscontrol section 28 with a data in (DI) signal. In addition, the logicsection 51 instructs the internal memory 52 to read and write, and theD/A converting section 53 to carry out its operation.

The nonvolatile memory in the internal memory 52 stores the correctiondata in digital values. The correction data are address mapped, and aretransmitted to the D/A converting section 53 sequentially in response tothe clock from the logic section 51. The D/A converting section 53supplies the constant current circuit 30 with the correction voltageobtained by the analog conversion of the digital values corresponding tothe correction data. Thus, the constant current corresponding to thecorrection voltage is supplied to the RS terminal.

FIG. 4 is a graph illustrating relationships between the current amountsupplied to the RS terminal and the correction light quantity of the LD21, in which the horizontal axis shows the RS supply current and thevertical axis shows the correction light quantity. The laser emissionlight quantity to be corrected is directly proportional to the currentamount supplied to the RS terminal.

FIG. 7 is a flowchart illustrating the control the exposure apparatuscontrol section 28 performs.

In response to a printing request at step S601, the exposure apparatuscontrol section 28 makes a decision about the light quantity setting forthe printing at the next step S602. For example, it makes a decision inaccordance with the printing speed and the designation of the lightquantity itself. The printing speed depends on the image formingapparatus using the exposure apparatus. The light quantity itself can bedesignated by the image forming apparatus side. The present embodimentselects and sets two types of light quantities in the light quantitysetting section 31 in accordance with the designation. The two types oflight quantities are light quantity setting 1; and light quantitysetting 2. FIG. 8 shows the nonvolatile memory data of the lightquantity correcting section 29 in this case. In FIG. 8, the unit of theRS supply current is μA, and the unit of pre-correction light quantityand post-correction light quantity is mW, which satisfy therelationships of FIG. 4. In the case of the light quantity setting 1,the data are assigned to addresses 0000h-001Fh; and in the case of thelight quantity setting 2, the data are assigned to addresses0020h-005Eh.

When the light quantity setting 1 is selected at step S602, the exposureapparatus control section 28 writes the light quantity correction startaddress “00H” into the light quantity correcting section 29 at stepS603, and writes the number of correction data “32” at step S604. At thenext step S605, the exposure apparatus control section 28 designates theclock for sequentially transferring the correction data to the D/Aconverting section 53 at 200 kHz.

In the present embodiment, since the number of data is 32 at 200 KHz,the light quantity correction is carried out for the duration 1/200k×32=160 μsec. The clock frequency and the number of correction data areusually optimized in accordance with the resolution of the lightquantity correction or to the rotation speed of the polygon mirror.

When the light quantity setting 2 is selected at step S602, the exposureapparatus control section 28 writes the light quantity correction startaddress “0020H” into the light quantity correcting section 29 at stepS606, and writes the number of correction data “63” at step S607. At thenext step S608, the exposure apparatus control section 28 designates theclock for sequentially transferring the correction data to the D/Aconverting section 53 at 200 kHz.

After completing steps S605 and S608, the exposure apparatus controlsection 28 waits until the scanner ready at the next step S609. The term“scanner ready” refers to a state in which the rotation rate of thepolygon mirror reaches the rotation rate enabling printing, and therotation becomes stable. For example, in response to the signal from therotation control apparatus of the polygon mirror indicating that therotation becomes stable, the exposure apparatus control section 28 makesa decision that it enters the scanner ready, and instructs the lightquantity correcting section 29 to start the light quantity correction atstep S610. Receiving the instruction, the light quantity correctingsection 29 carries out the light quantity correction with makingsynchronization at each scanning in response to the signal from the BDsensor. Subsequently, the exposure apparatus control section 28 makes adecision as to whether the printing has been completed or not at stepS611. If it makes a decision that the printing has been completed, itdelivers a light quantity correction stop instruction to the lightquantity correcting section at step S612, and terminates the control.

FIG. 6 illustrates this situations.

In the case of the light quantity setting 1, the resolution of the lightquantity correction is divided into 32 divisions in the main scanningdirection, and the correction is made in such a manner that the targetlight quantity becomes 1.2. In the case of the light quantity setting 2,the resolution of the light quantity correction is divided into 63divisions in the main scanning direction, and the correction is made insuch a manner that the target light quantity becomes 1.0. Theresolutions are determined in advance depending on the light quantity,and as the correction data, the values measured in advance for eachexposure apparatus are stored.

Although the present embodiment has two sets of the light quantitysetting values, it can deal with a greater number of sets.

As described above, the present embodiment has the correction datacorresponding to a plurality of light quantities. Accordingly, it canmaintain the light quantity on the photosensitive body surface nearlyconstant in the scanning direction by selecting the reference correctiondata even when switching the target light quantity.

Embodiment 2

In the foregoing embodiment, the data stored in the memory are subjectedto the digital-to-analog conversion without change. As a result, thevoltage output has the same resolution for the data without exception.

In the present embodiment, the relationships between the resolution andthe correction data are optimized before output.

The present embodiment will be described with reference to the drawings.

FIG. 9 is a table showing examples of data for carrying out correctionfor 40 data when the target light quantity on the photosensitive body is250 μW. The table shows calculation results for each correction voltagetarget (in mV), when the output reference voltage Vref is 75 mV and 10mV. As shown in FIG. 11, the output reference voltage Vref can bedesignated by 4-bit data, for example. Thus, it can be set at every 5 mVinterval from 0000B to 1111B. Accordingly, the highest output voltage,that is, the greatest correction amount is 75 mV, and the outputincreases 75 mV at every increment of the correction data. Incidentally,when the correction data length is set at 6 bits, the maximum outputvoltage at each Vref setting becomes as shown in FIG. 11.

FIG. 12 is a graph illustrating the correction voltage target and theoutput voltage (mV) when the Vref is set at 75 mV, and the outputvoltage (mV) when the Vref is set at 10 mV. When the Vref is set at 75mV, since the resolution for the correction data is coarse, the curvegreatly differs from the target. In contrast, when the Vref is set at 10mV, the curve is closer to the target output. In other words, it becomespossible to achieve more precise light quantity correction by settingthe Vref at the minimum possible value to determine the correction data,thereby being able to draw a more ideal correction data curve.

FIG. 13 is a flowchart illustrating the control performed by theexposure apparatus control section 28 of the present embodiment. Itdiffers from the control of the foregoing embodiment 1 only in stepsS706 and S710. Since the remaining control is the same, its duplicateexplanation will be omitted here.

At step S706, the exposure apparatus control section 28 selects the mode1 light quantity setting. FIG. 14 shows the relationships between theaddresses, memory data, and target light quantities. In FIG. 14, theunit of the RS supply current is μA, and the unit of the pre-correctionlight quantity and post-correction light quantity is mW. In the lightquantity setting 1, the target light quantity is set at 250 μW, and thememory data are stored for the Vref voltage of 10 mV. The exposureapparatus control section 28 writes “0010B” into the Vref settingregister to set the Vref voltage at 10 mV for the light quantitycorrecting section 29.

On the other hand, at step S710, the exposure apparatus control section28 selects the mode 2 light quantity setting. In the light quantitysetting 2, the target light quantity is set at 170 μW, and the memorydata are stored for the Vref voltage of 20 mV. The exposure apparatuscontrol section 28 writes “0100B” into the Vref setting register for thelight quantity correcting section 29.

Incidentally, such a configuration is also possible in which theexposure apparatus control section 28 stores the Vref voltagecorresponding to each light quantity setting in advance, and the lightquantity correcting section 29 employs the data corresponding to theVref values as its memory data. Alternatively, such a configuration ispossible in which the light quantity correcting section 29 stores in apart of its memory the Vref value corresponding to each light quantitysetting, and the exposure apparatus control section 28 reads the Vrefvalue every time the light quantity is changed, and instructs the lightquantity correcting section 29 as the Vref set.

Carrying out the foregoing control makes it possible to vary theresolution at the D/A conversion in accordance with the correctionamount. This enables more accurate light quantity correction in thescanning direction.

Embodiment 3

The foregoing embodiment stores the data in the memory by the number ofthe light quantity settings, and varies the light quantity irradiatedonto the photosensitive body by varying the correction light quantity bydesignating the data address to be read and the number of data.Accordingly, an increasing number of light quantity setting steps willincrease the number of memory data.

In contrast, the present embodiment handles a control method capable ofdealing with a plurality of light quantities with maintaining the numberof data in the memory at a single set.

FIG. 15 is a block diagram showing a configuration of the exposureapparatus of the present embodiment.

The present embodiment differs from the configuration of FIG. 2described in the foregoing embodiment in that it has an input terminalPref for setting the light quantity to the laser driver IC 27, and theremaining configuration is the same. The exposure apparatus controlsection 28 outputs a pulse-width modulation (PWM) pulse corresponding tothe light quantity setting value, and supplies the input terminal Prefwith the voltage smoothed by the resistor R4 and capacitor C2. The laserdriver IC 27 carries out the APC control with reference to the voltagesupplied to the input terminal Pref. Thus, the emission light quantityof the LD 21 is determined with reference to the voltage supplied to theinput terminal Pref. The duty of the PWM pulses is directly proportionalto the laser light quantity at the APC.

The light quantity setting in this case will be described.

Assume that it is necessary for the target light quantity on thephotosensitive body to be set at one of the three values 150 μW, 250 μWand 300 μW, for example.

In this case, the memory of the light quantity correcting section 29stores 20 data as shown in FIG. 16 as the light quantity correction datausing as the reference the case that carries out the light quantitycorrection with setting the target light quantity on the photosensitivebody at 250 μW.

Assume that the PWM duty when the target light quantity is 250 μW is50%, then the PWM duty when the target light quantity is 150 μW is setat 30%, and the PWM duty when the target light quantity 300 μW is set at60%. Varying the PWM duty in accordance with the changing rate of thetarget light quantity can vary the APC light quantity.

Accompanying this, the pre-correction light quantity at each targetlight quantity also varies in accordance with the changing rate of thePWM duty. The pre-correction light quantity corresponding to each targetlight quantity is shown in FIG. 16-FIG. 18.

Next, the designation of the voltage Vref at the correction will bedescribed. Consider the case where the correction is carried out becausethe pre-correction light quantity changes. In this case, if the voltageVref is fixed, the memory data used is the same so that the amount ofcorrection is the same, resulting in the unevenness of the lightquantity on the surface of the drum. In view of this, the voltage Vrefis set in accordance with the changing rate to the reference targetlight quantity 250 μW. More specifically, when the voltage Vref when thetarget light quantity is 250 μW is 10 mV, it is set at 6 mV when thetarget light quantity is 150 μW, and at 12 mV when the target lightquantity is 300 μW.

Since the correction voltage is the product of the voltage Vref and thedata, the correction voltage for each target light quantity isdetermined. On the other hand, the conversion of the correction voltageinto the correction current, and the conversion of the correctioncurrent into the light quantity are made at a fixed rate independentlyof the target light quantity. Subtracting the correction light quantityfrom the pre-correction light quantity by using the correction lightquantity thus obtained makes it possible to provide uniform lightquantity on the surface of the drum for each target light quantity.

The foregoing explanation can be described by the following expressions.PA=APCref−Pth

where PA is the reference target light quantity;

APCref is the laser light quantity on the photosensitive body beforecorrection at the reference case; and

Pth is the laser correction light quantity at the reference case;

To increase the target light quantity by a factor of 1.2:

$\begin{matrix}{{1.2 \times {PA}} = {1.2 \times ( {{APCref} - {Pth}} )}} \\{= {{1.2 \times {APCref}} - {1.2 \times {Pth}}}}\end{matrix}$

Thus, it is obvious that the APC light quantity is to be multiplied by1.2 and the laser correction light quantity is to be multiplied by 1.2.

Next, the clock will be described for transferring data to the D/Aconverter 53 in the light quantity correcting section 29. Generally, thereason for varying the target light quantity on the photosensitive bodyis that the laser irradiation duration per unit pixel varies as a resultof having to change the rotation speed of the polygon mirror owing tothe change of the printing speed of the image forming apparatus. To copewith this, it is necessary to change the number of data to the datalength corresponding to this, or to change the clock for transmittingthe memory data to the D/A converter in accordance with this.

This will be shown by the following expression.HCLK=(clock at reference time)×(polygon rotation rate afterchange)÷(polygon rotation rate at reference time)

For example, assume that the rotation rate of the polygon mirror at thereference light quantity is 25000 rpm, and the transfer clock of the D/Aconverter of the light quantity correcting section is 500 kHz. Besides,assume that the polygon rotation rate after the change is 20000 rpm.Then, the transfer clock after the change is 400 kHz.(500 kHz×20000 rpm÷25000 rpm=400 kHz)

In this way, the light quantity distribution on the scanning plane canbe made nearly constant in the following conditions for the single setof data and data length of the memory data. More specifically, themethod is possible which determines the correction voltage by changingthe APC light quantity and voltage Vref in accordance with the ratecorresponding to the target light quantities, and which changes the datatransfer clock of the D/A converter at the rate equal to the changingrate from the case of the polygon rotation rate that determines thememory data.

Although the foregoing description is made by way of example using thesingle set of memory data, this is not essential. It is obvious thatwhen the polygon rotation rate varies sharply, even if another set ofmemory data is stored to improve the correction accuracy, the amount ofthe memory data can be reduced from the amount of the memory data at apredetermined number of light quantities required.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2006-037106, filed Feb. 14, 2006, which is hereby incorporated byreference herein its entirety.

1. An exposure apparatus having a light source for emitting a light beamwith a light quantity corresponding to a current amount supplied, arotating polygon mirror with a plurality of reflection planes, therotating polygon minor rotating in a manner that the light beam emittedfrom the light source and reflected off a reflection plane scans on aphotosensitive body, and a beam magnifying unit for expanding the lightbeam emitted from the light source in a manner that the light beam isirradiated onto the rotating polygon minor with a beam width exceeding awidth of the reflection plane of the rotating polygon mirror in ascanning direction, comprising: a setting unit for setting a targetlight quantity of the light beam irradiated onto the photosensitivebody; and a current supplying unit for, if a first target light quantityis set by the setting unit, supplying the light source with firstcorrection current that will make a light quantity of the light beamirradiated onto the photosensitive body nearly constant in a mainscanning direction and, if a second target light quantity is set by thesetting unit, supplying the light source with second correction currentthat will make a light quantity of the light beam irradiated onto thephotosensitive body nearly constant in a main scanning direction.
 2. Theexposure apparatus as claimed in claim 1, wherein the current supplyingunit comprises: a D/A converter; a designating unit for designating anumber of transfer data or transfer clock on the basis of the targetlight quantity set by the setting unit; a data supplying unit fortransferring correction current profile data corresponding to the firstor second correction current set by the setting unit to the D/Aconverter in response to the information designated by the designatingunit; and a current circuit for supplying current to the light sourceaccording to the output of the D/A converter, the output being obtainedin response to the correction current profile data transferred from thedata supplying unit.
 3. The exposure apparatus as claimed in claim 2,wherein, if the target light quantity is relatively small, then thenumber of transfer data is larger or the transfer clock is smaller thanthose in a case where the target light quantity is relatively large. 4.An exposure apparatus as claimed in claim 1, wherein the setting unitsets the target light quantity of the light beam according to printspeed.
 5. An exposure apparatus as claimed in claim 4, wherein the printspeed corresponds to a rotating rate of the rotating polygon mirror. 6.An image forming apparatus having a light source for emitting a lightbeam with a light quantity corresponding to a current amount supplied, arotating polygon mirror with a plurality of reflection planes, therotating polygon minor rotating in a manner that the light beam emittedfrom the light source and reflected off a reflection plane scans on aphotosensitive body, and a beam magnifying unit for expanding the lightbeam emitted from the light source in a manner that the light beam isirradiated onto the rotating polygon minor with a beam width exceeding awidth of the reflection plane of the rotating polygon mirror in ascanning direction, comprising: a setting unit for setting a targetlight quantity of the light beam irradiated onto the photosensitivebody; and a current supplying unit for, if a first target light quantityis set by the setting unit, supplying the light source with firstcorrection current that will make a light quantity of the light beamirradiated onto the photosensitive body nearly constant in a mainscanning direction and, if a second target light quantity is set by thesetting unit, supplying the light source with second correction currentthat will make a light quantity of the light beam irradiated onto thephotosensitive body nearly constant in a main scanning direction.
 7. Theimage forming apparatus as claimed in claim 6, wherein the currentsupplying unit comprises: a D/A converter; a designating unit fordesignating a number of transfer data or transfer clock on the basis ofthe target light quantity set by the setting unit; a data supplying unitfor transferring correction current profile data corresponding to thefirst or second correction current set by the setting unit to the D/Aconverter in response to the information designated by the designatingunit; and a current circuit for supplying current to the light sourceaccording to the output of the D/A converter, the output being obtainedin response to the correction current profile data transferred from thedata supplying unit.
 8. The image forming apparatus as claimed in claim7, wherein, if the target light quantity is relatively small, then thenumber of transfer data is larger or the transfer clock is smaller thanthose in a case where the target light quantity is relatively large. 9.An image forming apparatus as claimed in claim 6, wherein the settingunit sets the target light quantity of the light beam according to printspeed.
 10. An image forming apparatus as claimed in claim 9, wherein theprint speed corresponds to a rotating rate of the rotating polygonmirror.